In automobiles and various vehicles, a vibration-insulating rubber is used as a material constituting the torsional damper, the engine mount, and the muffler hanger so as to absorb vibrations occurring when driving an engine, to thereby suppress noise. Such vibration-insulating rubbers for use in vehicles are required to have a vibration-insulating function of suppressing, by absorbing, vibrations of heavy load supported by the rubber, and strength properties capable of supporting heavy load.
Further, when used under a high temperature environment such as in an engine room, the rubber is also required to have high durability and settling resistance, not to mention being excellent in strength properties and vibration-insulating properties while having a low dynamic-to-static modulus ratio.
In particular, the temperature of an engine room tends to be increased in recent years along with high output of engine and reduction in engine room space due to increase in vehicle interior space, with the result that the rubber is often used in a severer environment. Therefore, there are strict requirements for the vibration-insulating rubber for use in automobiles regarding, for example, thermal resistance.
In general, to improve rubber in thermal resistance and settling resistance, it is effective to reduce the content of sulfur. However, the reduced content of sulfur deteriorates durability. To overcome such problems, for example, Patent Literature 1 discloses a technology of providing a rubber composition containing a low content of sulfur and zinc acrylate (ZAA). The technology of adding zinc acrylate (ZAA) or zinc methacrylate (ZMA) is also disclosed in Patent Literatures 2 and 3.
However, a vibration-insulating rubber to be obtained by the aforementioned technology is high in dynamic-to-static modulus ratio, and still far from satisfactory in terms of all aspects including thermal resistance, durability, and settling resistance. Thus, there has been a room for improvement in compounding of the rubber.
Meanwhile, Patent Literature 4 or corresponding Patent Literature 5 discloses a technology of using a rubber composition containing, as a crosslinking agent, bismaleimide having a specific chemical structure and sulfur, to thereby improve thermal resistance and a low dynamic-to-static modulus ratio.